It is well known to employ various intravascular endoprostheses delivered percutaneously for the treatment of diseases of various body vessels. These types of endoprostheses are commonly referred to as stents. A stent is generally a tubular device formed of a biocompatible material such as nitinol. The fabrication of stents from nitinol tubes that are cut by such methods as laser cutting, water jet cutting, electrical discharge maching, and chemical milling is commonly known. Nitinol is considered a shape memory alloy (SMA). Nitinol also has a shape setting temperature, defined as any temperature within the temperature range at which a shape memory alloy (SMA) article, when exposed to for a period of time in a constrained shape, will substantially maintain the constrained shape when the article is subsequently unconstrained.
The manufacturing of nitinol tubes is expensive. The larger the diameter of the nitinol tube the more expensive it becomes. The cost constraints of large diameter nitinol tubing have resulted in the practice of cutting patterns (such as stent patterns) into small diameter nitinol tubes and then incrementally expanding and shape setting these tubes to attain larger diameter nitinol tubes (and/or nitinol stents).
One general method of shape setting nitinol involves deforming and constraining the nitinol in a desired shape at room temperature (usually about 20° C.) or at below room temperature. The nitinol is then exposed to an elevated temperature (usually about 500° C.) while constrained in a desired shape, in a furnace for example, for a period of time (usually about 5 to 20 minutes). The nitinol is then cooled to room temperature by either water quenching or allowing the nitinol to air cool. This shape setting process imparts a new shape to the nitinol. The new shape is a result of the specific prior deformation and constraining of the cut tube.
In the case of expansion of a cut nitinol tube, a series of incremental expansion and shape setting steps are commonly used. The traditional method for nitinol stent device manufacturing is described by Poncin et. al. (SMST-2000 Conference Proceedings, pp 477-486) which states “[t]he device is expanded to its final size by a succession of progressive, shape-setting steps involving heat treatments.” Using a series of incremental expansion steps reduces the incidence of fracture or cracking of the cut nitinol tube during shape setting.
In one example, a stent pattern can be laser cut into a nitinol tube having an outer diameter of about 4 mm. In order to expand this 4 mm cut tube to a 24 mm cut tube, a series of incremental expansion steps would be taken. For example: the cut nitinol tube would be expanded from a 4 mm diameter to an 8 mm diameter and then shape set; next, the cut nitinol tube would then be expanded from 8 mm to 12 mm diameter and then shape set; and so on until the desired 24 mm diameter cut tube is attained.
It is common practice to utilize a series of expansion steps in stent forming to avoid stent fracture during the shape setting process. The above example utilized five expansion steps to attain the desired stent diameter of 24 mm. Omitting even one of these expansion steps, expanding from 4 mm to 12 mm and shape setting for example, can result in the stent fracturing during shape setting. This process of forming nitinol incrementally through a series of shape setting steps is costly and time consuming.
It is also common practice for those skilled in the art to cool nitinol stents, forming thermally induced martensite prior to the expansion of a nitinol tube. Nitinol tubes that are primarily austenite at room temperature will be easier to deform and diametrically expand if they are first cooled to form thermally induced martensite. Because martensitic nitinol is easier to deform than austenitic nitinol, it has been assumed that forming thermally induced martensite prior to expanding a nitinol tube will minimize crack formation in the stent. In spite of this practice of thermally inducing martensite prior to nitinol tube expansion, crack formation during expansion of nitinol tubes is a problem. The practice of thermally inducing martensite prior to nitinol tube expansion has not eliminated the need for incremental expansion steps required to diametrically expand and shape set nitinol tube.
Therefore, there has been a need to have a nitinol medical device forming process that overcomes the disadvantages of the prior art. The present invention provides such a solution.